When your Shadow Word: Pain damage over time critically strikes, you also create a shadowy version of yourself that floats towards the target and deals x Shadow damage.

Shadowy Apparitions is a passive priest ability learned at level 42 for those with the Shadow specialization. When dealing critical periodic damage with [Shadow Word: Pain], a shadowy version of the priest is summoned, floating towards the target while dealing damage.

Patch 5.4.0 (10-Sep-2013) Shadowy Apparitions now creates a shadowy version of the Priest that floats towards the target and deals damage. The apparitions are now treated as a missile, and is no longer an attackable creature.